Field of the Invention
The invention lies in the telecommunications field. More specifically, the invention relates to a method for measuring the characteristics of radio channels, in which the received signals are received by a number of receiving sensors in a two-dimensional antenna array, with the respective received signals being composed of wave elements of a transmitted signal with regard to the azimuth and elevation of different two-dimensional incidence direction and different delay.
The invention also relates to a measurement configuration for measuring the characteristics of radio channels having a planar antenna array, having a number of antenna sensors, in which case each antenna sensor is followed by analog/digital sampling, a filter matched to the signal and stage for discrete Fourier transformation, and at least one signal processor is provided for the reception stages.
In a large number of applications, such as sonar, radar, satellite communication and mobile radio, high-resolution radio channel measurements, which also supply direction information, are desirable. A radio channel represents, for example, the link between the base station and mobile stations. Increasing capacity requirements and limited frequency resources necessitate increased spectral efficiency. In this context, a noticeable improvement can be achieved by the use of intelligent antenna arrays at base stations, making use of the spatial diversity inherent in the radio channel. Two-dimensional array geometries allow estimation of both the azimuth and elevation angle of the dominant, that is to say the most powerful, for example, wave fronts which arrive at the array. For example 2D unitary ESPRIT is an efficient method, with an enclosed nature, for estimating automatically paired azimuth and elevation angles, as is described by Haardt, Zoltowski, Mathews, and Nossek, in "2D Unitary ESPRIT for Efficient 2D Parameter Estimation," in Proc. IEEE Int. Conf. Acoust., Speech, Signal Processing, vol. 3, 2096-99, Detroit, Mich. May 1995.
Detailed knowledge of the dominant azimuthal elevation angles for the mobile radio channel is essential in order to allow effective intelligent antenna concepts to be developed for future mobile radio systems. If the elevation of the dominant wave fronts is taken into account, this helps, for example, to considerably reduce the near-far problem in mobile radio systems as is particularly important, for example, for CDMA systems (Code Division Multiple Access) with direct sequences. Sophisticated concepts using intelligent antennas are of major interest, not least of commercial interest as well, for various mobile radio systems, in particular 3rd generation mobile radio systems.
It is thus essential to find and to evaluate the statistics of the two-dimensional incidence directions and delays of the arriving wave fronts.
Based on the fact that determination of only the azimuth of the arriving wave fronts results in a considerable error if the elevation of the arriving wave fronts is high, a 3D channel investigation method has been disclosed by Fuhl, Rossi, and Bonek, in "High-resolution 3D direction-of-arrival determination for urban mobile radio", IEEE Trans. Antennas and Propagation, vol. pp. 672-682, April 1997. In this reference, the authors describe a measurement configuration for channel investigation, in which high-resolution estimates of the 2D incidence directions (azimuth and elevation) of the dominant propagation paths are determined by solving a two-dimensional harmonic search problem separately for each estimated impulse-response delay time. The channel tester used in this case transmits a carrier at f.sub.0 =890 MHz, which is phase-modulated at 15 Mbits/s with a maximum length binary sequence which contains 511 bits. The measurements obtained on a uniform rectangular array of identical antennas are then demodulated, are mixed to baseband and are decomposed in order to obtain the I and Q components. The complex envelope curves obtained in this way are correlated with the maximum length binary sequence in order t o obtain the impulse response separately for each sensor in the antenna array. These impulse responses are stored for further processing. Both angles, namely the azimuth and elevation, of the dominant wave fronts arriving at the antenna array are determined jointly via the already mentioned 2D unitary ESPRIT, separately for each predetermined delay time. In this context, reference should also be made to M. Haardt, "Efficient One-, Two, and Multidimensional High-Resolution Array Signal Processing," Dissertation, Munich Technical University 1996, ISBN 3-8265-2220-6 (pages 63-66). Spatial smoothing with overlapping subgroups is used as a preprocessing step for decorrelation of coherent waves and, finally, the angles and amplitudes of the wave fronts are estimated separately for each delay time.
This 2D unitary ESPRIT method has been used successfully in various field-experiments using the channel tester and a uniform rectangular antenna array, with pairs of 2D incidence angle estimates being provided automatically for the dominant propagation paths. The measurement results form a basis for investigation and evaluation of beam tracking models and for the parameters of directional channel models. In most known measurement configurations, the (virtual) antenna array was installed at the base station, and the transmissions were made from the mobile station. In the cited reference IEEE Trans. Antennas and Propagation, the transmitter is provided at the base station and the (virtual) antenna array is mobile. In various known measurement trials, the antenna array has been simulated by moving an individual antenna to the positions of antenna elements in the virtual antenna array. In this case, particular attention has to be paid to exact positioning and careful synchronization between the transmitter and receiver, in which context reference is also made to U. Martin, "Modeling the Mobile Radio Channel by Echo Estimation," Frequenz, vol. 48, pp. 198-212, 1994. The reference which has already been referred to twice from IEEE Trans. Antennas and Propagation also shows that the use of the 2D unitary ESPRIT method leads to a resolution which is an order moving an individual antenna to the positions of antenna elements in the virtual antenna array. In this case, particular attention has to be paid to exact positioning and careful synchronization between the transmitter and receiver, in which context reference is also made to Martin, "Modeling the Mobile Radio Channel by Echo Estimation," Frequenz, vol. 48, pp. 198-212, 1994. The above-mentioned text from IEEE Trans. Antennas and Propagation also shows that the use of the 2D unitary ESPRIT method leads to a resolution which is an order of magnitude better than that for conventional Fourier analysis.
Furthermore, Roy and Kailath, in "ESPRIT Estimation of Signal Parameters Via Rotational Invariance Techniques," appearing in IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. 37, No. 7, July 1989, pages 984-995 disclose a method using which the incidence direction of received wave fronts can also be estimated. German published patent application DE 195 11 752 Al discloses a method for high-resolution evaluation of signals for direction or frequency estimation.
Josef Fuhl et al., "High-Resolution 3-D Direction-of-Arrival Determination for Urban Mobile Radio", IEEE Transactions on Antennas and Propagation, Volume 45, No. 4, April 1997, pages 672-682 describes a method for estimating the direction of electromagnetic waves arriving at a receiver in which, after previously determining the propagation time delays of the electromagnetic waves, the azimuth and elevation angles are determined at the same time.